Osirix v5

PV and PZ were semi-automatically segmented on axial fat-saturated T2WI MR images, in OsiriX v5.8.1 (Pixmeo, Geneva, Switzerland), as reported previously [21 (link), 22 (link)]. The radiologist (Q.Y.) with 10 years of experience in prostate MRI reviewed and manually corrected the contoured areas, under the supervision of a senior radiologist (J.L.) with over 30 years’ experience on prostate imaging. PV and PZ were calculated by multiplying the sum of contoured area on each slices by slice thickness (Supplementary Figure 1).

Skulls were CT scanned at the Pueblo Radiology Medical Group in Santa Barbara, California using a Siemans 16-slice Somatom Sensation 16 (1 mm slices, 100 Kv, 150 MAs, 380 mm FOV, soft tissue window, analyzed with bone algorithm on). Endocranial volume (cm³) was calculated using the DICOM viewer OsiriX v5.8.5 (32-bit, Pixmeo SARL, Switzerland; Figs. S1 and S2) for 1 mm cross-sectional slices (regular) and for 1 mm cross-sectional slices that were taken with the CT scanner bed moved 0.5 mm forward (offset), using the average endocranial volume $\left(\frac{\mathrm{regular}+\mathrm{offset}}{2}\right)$ in analyses. The offset was added to increase the precision of the endocranial volume measurements since grackle craniums are small (approximately 20 mm in length), resulting in about 20 slices per scan (one slice every 1 mm). The offset allowed us to measure more area (one slice every 0.5 mm) by increasing the number of slices to approximately 40 per skull.